For example, in a radio system installed outdoors and used for point-to-point communication networks, waveguides are used to establish connection between the radio device (transmitter and receiver) and the antenna device to transmit RF (Radio Frequency) signals such as microwaves, milliwaves and others. Most radio systems of this kind use linearly polarized waves, in different manners, including horizontally polarized waves, vertically polarized waves and 45-degree polarized waves as RF signals to be transmitted via the antenna device. Accordingly, there occur cases where the polarized wave of the RF signal that propagates through the antenna element and waveguide of the antenna device and the polarized wave of the RF signal that propagates in the waveguide of the radio device have different directions (horizontal, vertical and 45 degrees).
In order to make the directions of the polarized waves of the RF signals transmitted inside the radio device and the antenna device coincide, a well-known polarization converting circuit can be inserted between the radio device and the antenna device to change the directions of the polarizations. A configurational example of the polarization converting circuit is given in Patent Document 1, for example.
FIG. 1A is a side sectional diagram showing a configuration of a radio system related to the present invention. FIG. 1B is a schematic diagram showing sectional shapes of the radio system cut along a plane A-A′, plane B-B′ and plane C-C′ shown in FIG. 1A.
The related art radio system includes a radio device and an antenna device, each having a waveguide (not shown) and is configured so that the radio device and antenna device are connected and fixed to each other with the open ends of their waveguides abutted against each other. In the following description, the abutment of the radio device with the antenna device is called ‘device interface portion’ and the abutment of the antenna device with the radio device is called ‘antenna interface portion’. The waveguides provided for the radio device and the antenna device are fixed to the device interface portion and the antenna interface portion with their open ends exposed. FIG. 1A gives a configurational example of these device interface portion 101 and antenna interface portion 102 and their connection example.
As shown in FIG. 1A, polarization converting circuit 103 is set inside holder 104 arranged on the opening end side of the device interface portion 101 so as to be positioned between the waveguide of the radio device and the waveguide of the antenna device. The abutment faces of device interface portion 101 and antenna interface portion 102 are sealed with annular gasket 105 or the like so as to secure airtightness of the radio device and the antenna device installed outdoors.
FIG. 1B show examples of the shapes of the waveguides and polarization converting circuit 103, viewed from the opening end side, in device interface portion 101 and antenna interface portion 102. The rectangular hole inside each circle that represents the contour of the waveguide or polarization converting circuit 103 in FIG. 1B forms the transmission path of RF signals. In FIG. 1A, the hole that penetrates through device interface portion 101, antenna interface portion 102 and polarization converting circuit 103 forms a transmission path. “Device” shown in FIG. 1B represents device interface portion 101, “Conversion” represents polarization converting circuit 103 and “Antenna” represents antenna interface portion 102.
The radio system shown in FIGS. 1A and 1B shows an example in which the RF signal propagates as 45-degree polarized (θ=45 deg.) in the radio device and the RF signal propagates as vertically polarized (θ=0 deg.) or as horizontally polarized (θ=90 deg.). In this case, polarization converting circuit 103 is set at such an angle as to rotate the direction of the polarized waves without causing any reflection of the RF signal at the input and output ends. For example, when the RF signal propagates in vertical polarization in the antenna device, the rotating angle θ is set at +22.5 deg., whereas when the RF signal propagates in horizontal polarization, the rotating angle θ is set at +67.5 deg.
As shown in FIG. 1A, in the related art radio system, in order to secure airtightness of the radio device and the antenna device installed outdoors the polarization converting circuit is often built in the radio device. Therefore, the radio device and the antenna device are usually constructed such that the polarization converting circuit cannot be viewed from the outside.
Accordingly, the related art radio system entails the problem that the rotating angle of the polarization converting circuit cannot be confirmed at the time of, for example, installing the radio system after the radio device and the antenna device have been connected.
When the directions of the polarization of the RF signal differ between the radio device and the antenna device, there are some cases where the polarization converting circuit has been previously fixed at a predetermined rotating angle to the device interface portion, based on the relationship of polarizations between the radio device and the antenna device. In such a case, to avoid making any connection mistakes when connecting the radio device and the antenna device, it is necessary to carry out a task such as dismounting the antenna device in order to confirm the rotating angle of the polarization converting circuit, for example.